1. Technical Field
The present invention relates to an in-vehicle radar device that is installed in a vehicle and retrieves position information of a target in front of the vehicle, and in particular to phase correction of an antenna of the radar device.
2. Related Art
An in-vehicle radar device is an in-vehicle digital beam forming (DBF) radar device using adaptive array antenna technology where plural element antennas are arrayed and which changes the characteristics of the antenna by adaptively controlling the amplitude and phase of signals received by the element antennas. The in-vehicle radar device is installed in the front portion of a vehicle, emits a millimeter waveband radio beam or laser beam toward the front, receives the beam reflected by a target, and detects whether or not the target is present and position information (relative distance, direction, relative speed) to the target.
The basic principle of the generation and scanning of the antenna beam in such an in-vehicle DBF radar device will be briefly described by FIG. 11.
As shown in FIG. 11, a radio wave receiving antenna A comprises plural element antennas RX(1) to RX(n) arrayed in intervals d.
Assuming that a radio wave B of a wavelength λ is made incident from the outside on the radio wave receiving antenna A at an angle θ with respect to a front direction C of the element antennas RX(1) to RX(n) represented by the dotted line in the figure, a phase delay amount r(n) of the radio wave reaching each of the element antennas RX(1) to RX(n) is geometrically determined as in the following expression (1).RX(1): r(1)=0RX(2): r(2)=d sin θ/λRX(3): r(3)=2d sin θ/λRX(4): r(4)=3d sin θ/λRX(n): r(n)=(n−1)sin θ/λ  (1)
Here, each of the element antennas RX(1) to RX(n) receives the radio wave B at the same phase by advancing the phase of all the element antennas RX(1) to RX(n) the phase delay amount θ by DBF processing. Thus, the directionality of an array antenna 1 becomes oriented in the θ direction.
That is, by appropriately controlling this phase amount, it becomes possible to scan the antenna beam in a predetermined direction.
The above is the basic principle of the generation and scanning of an antenna beam by DBF processing.
However, an amplifier for amplifying the received signal of the radio wave receiving antenna is an active element, and when plural amplifiers are used, phase errors occur due to temperature changes and secular changes in the amplifier portions. Further, the tolerance of the element antenna disposed intervals that arises in the process of antenna manufacture becomes antenna phase error.
Because various error factors such as mentioned above are included, the phases r′(n) of the element antennas that are actually detected become as in the following expression (2).
Here, Δdn is inter-element antenna distance error, Δθn is flatness error that occurs due to warping and thermal expansion of a flat plate on which the element antennas are disposed, Φn is phase error resulting from the delay of active devices and circuit paths, and αn is phase error of active devices resulting from temperature characteristics and secular changes.r(1)′=(d+Δd1)sin(θ+Δθ1)/λ+Φ1+α1r(2)′=2(d+Δd2)sin(θ+Δθ2)/λ+Φ2+α2r(3)′=3(d+Δd3)sin(θ+Δθ3)/λ+Φ3+α3r(n)′=(n−1)(d+Δdn)sin(θ+Δθn)/λ+Φn+αn  (2)
In order to maintain inherent in-vehicle radar performance without the target detection precision dropping in the use environment of the in-vehicle radar device, it is necessary to correct initial phase errors and phase errors of the antenna and receiving system that occur in accompaniment with temperature characteristics and secular changes.
For this reason, in the in-vehicle DBF radar device described in JP-A-2002-162460, for example, a technique is employed where the phase errors of a reference element antenna and other element antennas are detected, and of the detected phase errors, the detected phase error of another element antenna belonging to the group including the reference element antenna is used as a reference, and the detected phase error is compared with an estimated phase error per element antenna belonging to the other group to determine a phase correction value. However, there are the problems that an element antenna serving as a reference is required, it is necessary to dispose a standard reflector in an azimuth 0 degree direction in front of the array antenna to detect initial phase error, and phase correction processing is not implemented unless the detected signal level resulting from the target is equal to or greater than a constant threshold.